The purpose of the proposed research is to elucidate in chemical terms the mechanism by which coagulation proteins interact to form multiprotein complexes which convert inactive zymogens to active enzymes. The system chosen for study is the formation and action of "prothrombinase", a complex composed of factor X, factor V, phospholipid and calcium, which converts prothrombin to thrombin. In particular, the focus has been on bovine factor V which interacts with each of the other five components of the system. Previous work in the past seven years has focused on the purification of factor v, its molecular forms, the kinetics and molecular changes brought about by four proteolytic enzymes which affect its activity, the interaction with factor XA, its binding to phospholipids, its content of calcium, the formation and inactivation of prothrombinase and most recently the role of carbohydrate residues in its activity. Substantial progress in all these areas has been made but further studies on bovine factor V are necessary to obtain a more complete and detailed picture. In addition, to extending these observations on bovine factor V, it is planned to begin a systematic study of human factor V where properties are known to differ from that of the bovine protein. Since this protein is subject to proteolytic attack by both thrombin and plasmin, isolation of fragments characteristic for each enzyme might allow more assessment of intravascular clotting and fibrinolysis. In addition, the interaction of factor V (both bovine and human) with platelets will be investigated since it is believed that the lipoprotein of the platelet membrane may be the localization of the forming prothrombinase complex. Further, it is not known whether the factor V associated with platelets is the same or different from plasma factor V. The inactivation or protection of factor XA by platelets will also be examined. Finally, selected other platelet membrane proteins will be examined. Further interrelationships with the hemostatis and plasma coagulation systems will be sought. BIBLIOGRAPHIC REFERENCES: Colman, R.W., Habel, M., Hollenberg, N.K., Birtch, A.G. and Busch, G.J. Hyperacute renal allograft rejection in the primate: Therapeutic limitations of antiplatelet agents alone and combined with heparin. Amer. J. Path. 82:25-42, 1976.